This invention relates to a liquid fuel metering and supply system for internal combustion engines generally, and more particularly relates to an improved fuel metering system comprising a single bifurcated metering system wherein fuel for all engine operating conditions is metered through a single calibrated orifice or jet in response to a proportional pressure reduction which results as the sum of a venturi pressure reduction and a novel auxiliary pressure reduction.
In prior carburetor art, much effort has been devoted to designing carburetor systems which are capable of accurately metering fuel in response to engine demand at all engine loads. Early in the development of the art, it was discovered that the principle of Venturi could be used to govern the relationship of air and fuel fluid flow in a carburetor system. Engine intake air, when channelled through a duct, would provide a pressure reduction proportional to the quantity of air passing therethrough according to the principle of Venturi. Ideally, this venturi pressure reduction could be used to act directly on liquid fuel to draw it through a calibrated orifice for metering the required proportional amount of fuel for engine operation at various speeds and loads. The practical realities of carburetor construction and engine operation, however, dictated that supplementary means would have to be compounded with the venturi pressure reduction ideal to provide the correct amount of fuel for the various engine operating conditions.
Three things, in particular, served to complicate the basic situation. First, an internal combustion engine requires a numerically higher fuel/air ratio (richer mixture) when idling and operating at light load than when operating at larger loads. This is due, in part, to dilution of the intake fuel/air charge with exhaust gas as a result of overlap in valve timing. A simple venturi metering system cannot provide the necessary light load mixture enrichment. Secondly, the usual carburetor construction employs a constant level fuel reservoir to make fuel available to the metering orifice and to establish a pressure reference for the venturi pressure reduction. Any duct or conduit metering liquid fuel from the fuel reservoir to the air intake must typically pass above the plane of the fuel level to ensure that fuel does not drain or siphon into the air inlet in the absence of the proper metering pressure reduction. A significant supplementary pressure difference is required to raise fuel in the conduit substantially above the plane of the reservoir fuel level. Third, at small engine loads the speed of the intake air through the intake air duct is insufficient to provide good atomization of liquid fuel delivered therein, as would be the case with a simple venturi metering system.
The small magnitude of the venturi pressure reduction at small engine loads makes it very difficult to achieve proper fuel metering for all engine loads using a venturi metering system. Ideally, a venturi pressure signal would provide a substantially constant fuel/air ratio for all load conditions. However, the constant impedance to fuel flow caused by the pressure head of the metered fuel flowing to a conduit above the fuel level interferes with the idle load fuel metering. Under load, the venturi pressure reduction signal has a much larger magnitude than at idle, as the pressure required to raise the fuel above the fuel level may be insignificant insofar as its effect on proper fuel metering. Thus, a carburetor utilizing essentially only a venturi pressure reduction to meter fuel for all engine loads could provide a substantially constant fuel/air ratio whenever the engine was under load, but not for an idling condition. At idling conditions, the fuel/air ratio will be somewhat leaner than the substantially constant fuel/air ratio provided when the engine is under load. To optimize fuel metering under these conditions, the fuel level must be at, or just slightly below, any fuel metering conduit; and, in any case, any fuel metering conduit cannot be substantially above the fuel level.
Thus the small magnitude of the venturi pressure reduction at light engine loads makes it very difficult to achieve proper fuel metering using a simple venturi metering system. Attempts at such carburetor designs necessitate impractical small elevations of fuel above the fuel plane, and complicated arrangements of nozzles and chambers to ensure fuel delivery at all engine loads.
As a result of these problems, many carburetion systems evolved using supplementary fuel metering and supply circuits to augment fuel delivery over that provided by a circuit utilizing the basic venturi pressure reduction fuel supply ideal. Most of these were fuel metering for the various engine load conditions. At the time of this writing, the most common construction employs a basic venturi-operated circuit for large load fuel supply, and separate idle and off-idle transfer circuits for supplying fuel to the engine at smaller loads where the venturi pressure reduction is insufficient to cause fuel to be raised aboe the fuel level into the large load circuit. The idle circuit uses engine induction vacuum downstream of the throttle to provide the pressure difference for metering idle fuel, and the off-idle transfer circuit is progressively exposed to induction vacuum as the throttle valve opens. The use of separate circuits to meter and supply fuel for different engine load conditions necessitates extremely careful design of each circuit, and presents the problem of considerable undesirable variation in the fuel/air ratio when carburetor operation is transistioning from one circuit to another.
A solution to the problems of multiple circuit designs would seem to be to design a carburetion system having basically a single metering circuit which would somehow respond to different intake airflows to meter the correct amount of fuel for all engine load conditions; not have undesirable fuel/air ratio variations; and still be adjustable to suit various applications.
One previous design attempt having a single metering system uses a venturi pressure reduction to cause fuel to flow from a fuel reservoir to a nozzle located very close to an opening which is adjacent the intake air duct. The nozzle directs the metered fuel to a system of ducts and nozzles which deliver the fuel to the intake air downstream from a throttle valve. The intended advantage of the design is to deliver all fuel to the intake air at a location where the air turbulence and speed is sufficient to atomize the liquid fuel, downstream of the throttle valve. The design, however, requires a precisely shaped fluidic device having a carefully spaced interaction zone located in very close proximity to the air intake duct. These special shapes and locations render the design difficult to construct. Additionally, to make use of the venturi pressure reduction at small engine loads, the height that the metered fuel is raised above the plane of the fuel level must be kept as small as possible. This necessitates impractically close control of the fuel level. Finally, the design affords no adjustability from the idle fuel/air ratio.
Another known design meters fuel in a single circuit by employing mechanically variable valves to control the speed of the air in the intake duct and the amount of fuel metered in response. In this design, the intake duct is of rectilinear cross-section and has one wall movable and pivoted so it can vary the cross-section of the intake duct. This movable wall closes the intake duct to a small cross-section at small engine loads, so that the venturi pressure reduction is enchanced due to the relatively greater speed of the intake air. The fuel metering orifice is also of variable cross-section, having a profiled metering needle mechanically coupled to the movable intake duct wall such that the relationship of the openings at any engine load allows the proper amount of fuel to be metered. Since this type of carburetor has a mechanically variable intake duct forming the venturi, it has come to be known in the art as a variable venturi carburetor. Variable venturi carburetors are capable of very precise fuel metering if the needle valve profile is accurately designed, and have the additional advantage of having a strong enough venturi pressure reduction to meter fuel at all engine loads, including idle. The variable venturi carburetor has disadvantages, however, in that its mechanical complexity renders it expensive to construct and prone to mechanical breakdown. Additionally, the precision tolerances involved in the moving venturi and needle valve not only increase the cost of manufacture, but also subject the carburetor to malfunction in the presence of foreign particles or deposits which normally occur over a period of time.
The present invention provides a carburetor which avoids the problems inherent in the previously mentioned known carburetor designs. According to the present invention, there is provided a carburetor of the fixed venturi type, having a single bifurcated metering system comprising a large load circuit or passageway, and a small and medium load circuit or passageway. Since the present carburetor is of the fixed venturi type, the drawbacks due to the mechanical complexity of the variable venturi carburetor are avoided. The present carburetor employs a single metering system which meters fuel through a single calibrated orifice in response to a single net pressure reduction at all speeds and loads, thus avoiding the transition problems of designs having more than one metering system. The metering system of the present carburetor is bifurcated into two circuits or passageways: a small and medium load circuit to deliver fuel downstream of the throttle, and a large load circuit to deliver fuel to the venturi upstream of the throttle. Fuel flow through the two circuits adjusts automatically in response to engine operating conditions. The use of these two circuits ensures better atomization of the fuel at all engine loads. At idle, essentially all fuel is delivered to the intake air downstream of the throttle, where the very high turbulence assures good mixing of the fuel with the air. At large or maximum loads, however, there is little more turbulence downstream from the throttle than upstream. Intake air speed, though, is highest in the venturi at large loads. Therefore, the present invention provides a circuit which delivers large and maximum load fuel to the intake air at the venturi, where the high air speed gives much better fuel atomization than if the large load fuel entered the intake air downstream from the throttle. This provides significantly better large load fuel atomization than designs where large load fuel enters the intake air downstream from the throttle.
Also, a novel auxiliary pressure reduction in the present invention permits fuel to be raised as high as desired above the plane of the fuel level, thus avoiding the drawbacks of previous designs which require this height to be insubstantial.
It has long been recognized in the carburetor art that engine efficiency is enhanced as the homogeneity of the fuel/air mixture is improved. This effect reaches a maximum when the liquid fuel can be completely vaporized for forming the fuel/air intake charge prior to combustion. An engine intake charge so formed allows for an optimal fuel/air ratio and provides excellent cylinder-to-cylinder fuel distrubution characteristics in a multi-cylinder engine. The overall result is very favorable engine operation with respect to fuel consumption and undesirable engine exhaust emissions.
Prior efforts at liquid fuel vaporization generally involved metering the liquid fuel into the engine intake air, subsequently heating the fuel/air mixture, and then delivering the fuel/air mixture to the engine. This method involved extensive rerouting of the intake charge, resulting in mechanical complexity and overall bulk. The method could also be dangerous, in cases where engine exhaust provided vaporization heat and temperature of the fuel/air mixture approached the point of ignition.
There were also attempts to vaporize liquid fuel prior to mixing it with the intake air. The difficulties in vaporizing the fuel without disturbing the relatively small metering pressures make this method very difficult. Such attempts were characterized by complexity, often employing many valves and other mechanical controls to coordinate complicated networks of fuel and air passages.
As a result of their complexity, these previous vaporizing methods were impractical both in construction and operation and never enjoyed common application.
The present invention lends itself well to application of various fuel vaporization techniques, particularly the technique of using engine heat to vaporize the fuel before delivering it to the air intake. Due to the large pressure difference across the low and medium load fuel delivery circuit of the present invention, fuel can be vaporized without detrimental effect on the pressures required for precise fuel metering.